This invention will be described primarily by reference to specific embodiments useful with devices for dispensing air-treating vapors, such as air fresheners and insecticides, by evaporation and/or migration of air-treating substances through retention materials. The elapsed time indicator of this invention has particular usefulness in such applications, but it is to be understood that this invention has a number of other applications.
A great variety of devices have been developed to dispense air-treating vapors. Some of such devices dispense the vapors of an air-treating composition which is contained within and/or behind a polymeric material through which the air-treating composition migrates until released as a vapor at an outer surface. One product of this type includes perfume-impregnated composites of ethylene and polar monomer copolymers, such as vinyl acetate and ethyl acrylate. (See, for example, U.S. Pat. No. 4,095,031, entitled "Perfume Copolymers of Ethylene and Polar Monomer," issued in the name of Edward Jacob Engle on June 13, 1978.) Such plastic composites may be formed into beads which can be made to tumble in a rotatable vented container to emit air-treating vapors.
One problem or drawback in products of this type is that the product user cannot easily determine when the available air-treating composition is sufficiently depleted to require replacement. Therefore, it is desirable to have some sort of means to prompt replacement.
A variety of elapsed time indicators have been disclosed, including the following:
U.S. Pat. No. 3,018,611 discloses a complex device having an air inflow restriction and an oxygen-reactive material which gradually changes in color when it comes in contact with inflowing air. A predetermined period of time is known to have elapsed when the color of the strip has changed sufficiently.
U.S. Pat. No. 3,480,402 discloses a time indicator having an absorbent carrier holding a chemical compound which changes color upon exposure to oxygen. The compound is protected from the atmosphere by a layer through which atmospheric oxygen can be controllably diffused over a predetermined period of time.
U.S. Pat. No. 3,520,124 discloses the use of chemical or physical reactions of two materials to produce an abrupt color change when the reaction is complete. The reaction is initiated by removal of a barrier.
U.S. Pat. No. 3,996,007 discloses a time-temperature indicator having a rate-controlling film formed into a sealed pouch containing a gas-generating means in an ampule surrounded by a porous protective layer. The gas may be generated by vaporization, sublimation, or otherwise. When the gas is released, it reacts with an indicator composition to produce a color change.
U.S. Pat. No. 4,028,876 discloses apparatus for visually indicating elapsed time by a color change occurring on the reaction of two chromophoric compounds that are combined with one another through a matrix. The color change is gradual, rather than abrupt, and color comparisons must be made to judge when a predetermined time period has passed.
U.S. Pat. No. 4,128,508 discloses a color change perfume system using an acid or base which volatilizes at about the same rate as the perfume. The volatilization of the acid or base causes a pH indicator to change color as the perfume is exhausted.
U.S. Pat. No. 4,137,049 discloses a complex elapsed time indicator including two reactive chemicals, a pH change indicator, and a container which houses a frangible capsule and a porous carrier. The capsule holds a vapor-generating chemical. The porous carrier is impregnated with the other reactive chemical, the pH indicator, and a gel-forming agent. Vapor from the vapor-generating capsule gradually moves through and reacts in the carrier causing a gradual color change.
U.S. Pat. Nos. 4,195,055 and 4,195,057 disclose vapor-phase moving-boundary time indicators in which vapor permeating through a porous substrate which is coated with an indicating solid provides a visible moving boundary between two colors.
U.S. Pat. Nos. 4,195,056 and 4,195,058 disclose time-temperature indicators of a type in which a vapor-permeable barrier is positioned between a vapor and an indicator, all within an impermeable container.
U.S. Pat. No. 4,212,153 discloses a laminated indicator which utilizes the migration of a dye or the migration of an acid or base to interact with a pH indicator The color change occurring in this product is gradual, rather than abrupt.
U.S. Pat. No. 4,229,813 discloses an indicator which uses the migration of silicone oil along an absorbent strip to measure elapsed time. When the oil makes contact with ink, the color of the strip is changed. The device is said to be substantially unaffected by temperature.
U.S. Pat. No. 4,248,597 discloses a device having a pH color indicator. A color change occurs as the concentration of a removable substance having acidic or basic groups within a porous matrix diminishes by passing through a permeable film.
There are a number of disadvantages with devices of the prior art. For example, many of the prior elapsed time indicators fail to provide a sufficiently abrupt indication that a predetermined time period has elapsed. An abrupt color change is generally preferred over a change which is gradual over a long period of time. Gradual changes often require comparisons against a color standard to judge when sufficient time has elapsed. Many time indicator devices of the prior art are often too temperature or humidity sensitive to be useful under widely varying temperature and humidity conditions. Many devices of the prior art are very complex and expensive, and not given to simple use with vapor-dispensing products.
There is a need for a simple, reliable elapsed time indicator which can be used with vapor-dispensing products of various kinds.